1. Field of the Invention
The present invention relates to radiation detection and, more specifically, to methods for improving the accuracy of radiation contamination monitors used to assess surface contamination on the body of workers.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Given the potentially harmful physiological effects from exposure to radiation, it is important to regularly monitor radiation workers, and other workers with potential whole-body exposure to radioactive materials, to assess potential contamination that might be present on or within the worker's body. Whole-body ionizing radiation-monitoring devices, such as that depicted in FIG. 1, have been developed to automate and to improve the efficiency and effectiveness of this monitoring process. As depicted, the monitoring portal (102) utilizes an array of radiation detectors (104), a number of which detect gamma radiation. The array (104) allows a radiation worker (106) to enter the device and stand with one side of the worker's body positioned before the detector array (104). An arm, side leg, and hand monitoring array (108) is also present in the depiction. As shown, the worker is facing the detector array (104), with his or her right hand and arm inserted in the array (108). Detectors (110) on the base of the monitoring portal detect contamination that may be present on the worker's feet. Thus, the worker's anterior surface, right arm, right outside leg, and feet are being monitored for surface contamination. Once this anterior surface monitoring is complete, the worker turns with his or her posterior surface against the detector array (104), with his or her left hand and arm inserted in the array (108). Once the posterior surface has been surveyed, the measurement data is processed by an automated computing device, which logs the measurements and signals an alarm if necessary.
On-site calibration of such detection devices is crucial to the accuracy of its radiation measurements. As is commonly understood, gamma-ray background radiation is always present at levels that vary from physical location to location. To compensate for this background radiation variability, an on-site calibration routine utilizing one or more workers—who are known not to be contaminated—is typically performed to determine the existing background radiation properties and their effects on occupant self-shielding. The predetermined calibration factors to calculate the effects caused by the fact that the worker's body has a shielding effect on the detector array (104) may not fully describe the self-shielding on-site.
When standing in front of the detector array (104), the worker's body will absorb and/or scatter a certain amount of the gamma background radiation. This causes the resulting net count to be different than the actual count present on the worker's body, because the background radiation (though no longer being detected at the same rate as before the worker entered the device) is still being deducted from the measured counts. Thus, the sensitivity and accuracy of the device is negatively impacted by this effect.
A common method currently employed in an attempt to deal with this problem is to apply a single correction factor to the measurement that compensates for this whole-body shielding effect upon the detectors. Essentially, a group of workers who are known not to be contaminated is surveyed and an average “standard” worker profile is obtained. This “standard” worker profile is intended to compensate for the average shielding effect that a body has on the measured background radiation. Thus, a single correction factor is applied to the count rate determinations. While this is an improvement over previous methods of count rate determinations, it still does not account for the effects of varying body shapes and sizes. For example, a worker with a slight frame (less shielding than average) will experience a net count rate greater than actual while a worker with a heavier frame (more shielding than average) will experience a net count rate less than actual.
Moreover, the average “standard” worker profile does not compensate for the effects that various body heights and thicknesses have on the sensitivity and accuracy of the various sections of the detector array. For example, to increase sensitivity in the detectors, various detectors within the array will be grouped together to form a larger effective scintillating volume. Therefore, a worker that is shorter than average or that has a slight frame will leave certain detector groupings exposed to (or unshielded from) the background counts. Thus, while the average “standard” worker profile compensation assumes that these certain detector groupings should be experiencing some shielding, the resulting net count rate will be greater than actual. Conversely, a worker that is much taller than average or that has a much heavier frame will provide more shielding than average in a greater number of detector groupings. Because shielding for this worker is more than average, the resulting net count rate will be less than actual.
Further still, this average “standard” worker profile does not account for varying thickness of individual workers. For example, a worker with an athletic build (small waist and gut area, but large upper body) will provide more shielding to portions of the detector array near the chest area as opposed to those near the waist area. Thus, the portions of the detector array near the chest area, in reality, require a different correction factor than those near the worker's waist. This requirement is reversed in the situation with a worker having an excessively large midsection and relatively small upper body. Accordingly, the current “one-size-fits-all” approach does not adequately compensate for varying worker bodies.
Still others have attempted to compensate for this shielding effect by considering the worker's body weight in determining a correction factor. While this may give some indication as to the worker's density, it is no more accurate because it fails to consider the overall density distribution. Different types of tissue (muscle, adipose, etc.) have different densities and elemental compositions, affecting their absorption and scattering properties. Again, a worker with an athletic build may carry all of his or her weight in the chest region, yet would weigh the same as (or even more than) an obese worker carrying all of his or her weight in the gut. Moreover, a 183 cm tall individual that weighs 80 kg will have a different density distribution than a 152 cm tall individual of the same weight. Again, the same poorly-corrected-for shielding problems occur. What is needed is a more accurate method of correcting for the self-shielding effect that varying body sizes have upon whole-body detector arrays.